This invention relates to demagnetizing apparatus for electromagnets and workpieces held thereby and more particularly to such apparatus which is capable of demagnetizing a plurality of workpieces so as to allow removal of each workpiece without affecting the other workpieces.
In many environments in which an electromagnet is used for holding a workpiece, it is necessary to provide some means for demagnetizing the electromagnet and a workpiece held thereby before the workpiece can be readily removed from the holding magnet. Otherwise, the residual magnetism left in both the magnet and the workpiece may be sufficient to inhibit easy removal of the workpiece from the magnet. One example of such a situation is where an electromagnetic chuck is used for holding one or a plurality of workpieces for a grinding operation.
U.S. Pat. No. 4,370,693, assigned to the assignee of the present invention and incorporated herein by reference, discloses a demagnetizing apparatus for a workpiece-holding electromagnet. This demagnetizing apparatus includes switching means which can switch the magnetic hold of the electromagnet between various states of magnetization corresponding to full, variable, residual, and zero levels of magnetization.
The embodiment disclosed by U.S. Pat. No. b 4,370,693 specifically includes four switch positions designated FULL, VARIABLE, RESIDUAL and RELEASE. In the FULL switch position, maximum rectified a.c. power is transmitted to the electromagnetic chuck winding in order to provide the greatest holding power for the workpiece. When the switch is in the RESIDUAL position the holding power of the electromagnet is reduced, but not demagnetized, to a level that would allow one of a collection of workpieces to be removed when a multiplicity of workpieces are being machined or ground down at one time.
In some cases, it may be desirable to firmly hold a smaller workpiece without exerting FULL holding power, e.g., where such power might distort the workpiece. The VARIABLE setting is provided for this purpose. This switch position provides a preselected level of magnetization, the level typically selected to be less than FULL but substantially greater than the so-called RESIDUAL setting.
If it is desired to completely release the workpiece or workpieces, the control switch S1 may be moved to the RELEASE position to effect automatic demagnetization of the electromagnetic chuck and any permeable workpiece held thereby.
The RESIDUAL setting does not, however, always provide sufficient magnetism to hold the workpiece in place. When the chuck control is switched from a low level of VARIABLE holding to the RESIDUAL mode, the resulting residual magnetic holding force may be less than the minimum holding force intended to be supplied by the RESIDUAL mode. As a result, an operator may have handling problems with the workpieces since the non-maneuvered workpieces may shift.
Among the several objects of the present invention may be noted the provision of such an apparatus which will insure that a chuck will always achieve maximum residual magnetic holding force when in the RESIDUAL mode; the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.